This invention relates to integrated circuit fabrication tools and processes and, more particularly, to a method and apparatus for cleaning a pellicled reticle.
Integrated circuits (IC) commonly are fabricated on a semiconductor wafer. The semiconductor wafer typically is subjected to doping, deposition, etching, planarizing and lithographic processes to form semiconductor devices in the wafer. The wafer typically is cut to form multiple semiconductor "IC chips". Each chip includes many semiconductor devices. Although the label semiconductor is used, the devices are fabricated from various materials, including electrical conductors (e.g., aluminum, tungsten), electrical semiconductors (e.g., silicon) and electrical non-conductors (e.g., silicon dioxide).
A reticle is used in a lithographic process to define a photomask. A lithographic process refers to a process in which a pattern is delineated in a layer of material (e.g., photoresist) sensitive to photons, electrons or ions. The principle is similar to that of a photocamera in which an object is imaged on a photo-sensitive emulsion film. While with a photo-camera the "final product" is the printed image, the image in the semiconductor process context typically is an intermediate pattern, which defines regions where material is deposited or removed. The lithographic process typically involves multiple exposing and developing steps, wherein at a given step the photoresist is exposed to photons, electrons or ions, then developed to remove one of either the exposed or unexposed portions of photoresist. Complex patterns typically require multiple exposure and development steps.
A typical lithographic system includes a light source, optical system and transparent photomask. The light source emits light through the optical system and photomask onto a photoresist layer of a semiconductor wafer. The photomask defines the "intermediate pattern" used for determining where photoresist is to be removed or left in place. Conventional photomasks are transparent masks. A photomask typically is formed on a glass blank. The mask and blank together are referred to as a reticle. Conventional materials for the blank include soda lime, borosilicate glass or fused silica. The photomask is formed by a film of opaque material. Typically, the film is formed with chrome less than 100 nm thick and covered with an anti-reflective coating, such as chrome oxide. The purpose of the anti-reflective coating is to suppress ghost images from the light reflected by the opaque material.
The photomask serves to define geometries for materials deposited or etched on the wafer or materials applied to the wafer. The patterned film on the reticle blank includes mask lines and line spacings of less than 10 microns. Depending on the reduction factor x, line width and line space geometries for a resulting semiconductor device are from less than 10 microns to less than 2 microns. Other mask line spacings and semiconductor line spacings also can be achieved. When working with such small geometries, it is important that the reticle and other components in the fabrication processes be free of foreign particles. A tiny speck of dust alters the desired pattern to be imaged onto the wafer. Conventionally, a thin transparent membrane, referred to as a pellicle membrane, is applied over the photomask portion of the reticle to keep the photomask portion free of foreign particles. The pellicle membrane typically is positioned at a height above the photomask. Such height is greater than the focal length of the light imaged onto the photomask. Thus, small particles on the pellicle membrane will not block light from reaching the photomask.
Another problem caused by foreign particles is bad registration of the reticle. During a lithographic process, the reticle rests on a reticle table. The reticle table typically is part of a stepper device, which also includes a light source and a stepper control. The stepper control manages the relative position of the light source and the reticle table. Even the smallest of particles on the edge of the reticle can lift a portion of the reticle off the reticle table. Such offset of the reticle can result in bad registration of the light onto the wafer, which, in turn, can result in bad overlay from one pattern to another. Because the pellicle membrane typically is very fragile, the pellicle membrane is destroyed during the course of cleaning the reticle. Conventionally, the pellicle membrane is removed, the entire surface of the reticle is cleaned, and then the reticle undergoes requalification. Such a process is very time consuming and costly. Accordingly, there is a need for an alternative method and apparatus for cleaning a reticle.